Impact protection for plastic tubs employing mold retention rib

ABSTRACT

The decreased impact resistance of a unipartite plastic tub formed to include internal mold retention ribs is compensated for by the addition of external ribs, substantially coextensive in length and position with the internal ribs, having a width greater than that of the internal ribs to spread the stress concentration area beyond the juncture of the internal ribs and tub wall and thus avoid the conventional &#39;&#39;&#39;&#39;notch&#39;&#39;&#39;&#39; effect.

United States Patent [191 Butsch '1, 3,822,029 [451 July 2,1974

[ IMPACT PROTECTION FOR PLASTIC TUBS EMPLOYING MOLD RETENTION RIB [75] Inventor: Richard L. Butsch, Louisville, Ky.

[73] Assignee: General Electric Company,

Louisville, Ky.

[22] Filed: Dec. 29, 1972 [2]] Appl. No.: 319,557

[52] US. Cl. 220/72, 134/200 [51] Int. Cl B65d 7/42 [58] Field of Search 220/72, 71, 73, 84;

134/200, 165,56 D, 57 D, 58 D, 58

[56] References Cited UNITED STATES PATENTS LINES OF MAXIMUM STRESS l/l939 Harper 220/72X 3,064,6 6l 11/1962 Jacobs l34/58D Primary Examiner-William 1. Price Assistant Examiner-Steven M. Pollard Attorney, Agent, or Firm--Francis H. Boos [57] ABSTRACT The decreased impact resistance of a unipartite plastic tub formed to include internal mold retention ribs is compensated for by the addition of external ribs, substantially coextensive in length and position with the internal ribs, having a width greater than that of the internal ribs to spread the stress concentration area beyond the juncture of the internal ribs and tub wall and thus avoid the conventional notch effect.

5 Claims, 6 Drawing Figures IMPACT PATENTEDJUL 2 m4 FIGS . IMPACT PROTECTION FOR PLASTIC 'IUBS EMPLOYING MOLD RETENTION RIB BACKGROUND OF THE INVENTION The invention is directed to large capacity front loading dishwashers of the type normally comprising a permanent installation and employing upper and lower dish supporting racks as opposed to the smaller, low capacity, portable and/or top loading dishwashers. The reason for the distinction is that the ultimate objective of the invention is the employment of a unipartite plastic tub in large capacity, front loading dishwashers and while some of the problems presented by a proposed use of such a tub would be common to dishwashers of either type; the majority are distinct. Typical distinctions relate to increasing wall flexibility with increasing area, the absence of encompassing metal housings, greater tub shape complexity and additional interior tub mountings such as dual rack support structures. Accordingly, the ensuing background discussion relates primarily to front loading dishwashers of the type normally adapted for a permanent, under cabinet, installation.

The high temperature, corrosive environment to which automatic dishwasher chambers are subjected dictates that the surfaces of those internal components in contact with such environment be either highly resistant, or immune, to corrosion. Corrosion resistant metals such as stainless steel represent a substantial cost factor in this highly competitive appliance field and the long term industry trend has been toward the use of inert materials to resist the corrosive effects of the chamber environment which are magnified adjacent the usual metal tub seam welds. Insofar as the tub itself, representing the far greater internal surface area expo sure, is concerned; the general trend in the United States has been toward the application of an adherent plastic coating to the tub interior while European manufacturers have gone more to the use of plastic tub liners. In either event, the basic structural and design parameters that have prevailed since the advent of the modern front loading automatic dishwasher remained valid. This for the reason that overall dishwasher design parameters, both as regards utility and structural integrity, are virtually unaffected by whether the conventional metal tub is coated, lined or exposed.

Conceptually, the use of a unipartite plastic dishwasher tub, i.e., one not requiringa more rigid backing or outer support structure, represents a substantial advance in the art. Exemplary are the greatly decreased cost factors of a molded plastic tub vis-a-vis that of a metal tub requiring a further coating or lining operation; reduction in finishing labor requirements and particularly tub welding operations; longer tub life due to corrosion resistance which is independent of faulty coating procedures or chipping; decreased thermal and acoustical transmission thus minimizing the need for separate insulation; and a lesser inventory requirement for separable components susceptible of integral molding with the tub.

The reduction to practice of such a concept, as applied to front loading dishwashers on an assembly line basis, however, presents a myriad of problems which arise as a requirement for different approaches to those engineering considerations which had previously been predicated upon the use of a metal tub. The distinctions are primarily those associated with material strength the unusually high temperature environment to which the tub is subjected and considerations relating to plastic molding procedures. Thus, while the structural integrity of a metallic box beam shape defined by the conventional front loading tub is more than sufiicient to resist excessive wall deformation as a function of normal loading forces as during installation, adjustment or use and temperature variations. from room ambient to approximately 255F.; such normally applied loading forces would, in the absence of appropriate compensating structure, produce unacceptable wall deformation in a like configured plastic tub. The use of strand-like reinforcing fillers such as fiber glass would, seemingly, ameliorate the overall problem of wall deformation. In actuality, however, the presence of such reinforcing fibers not only effect tub discoloration as discussed below, but their random distribution precludes an accurate prediction of thermal growth characteristics which is an essential parameter in the design of certain compensating structure. Similarly, impact strength is normally of little moment in the design and installation of metallic tubs but is a matter of major concern in plastic tub design. Furthermore, various structural features appearing in a formed plastic tub as an incident of the forming operation create problems not previously associated with metal tub fabrication. Exemplary of the latter distinction are the presence, on the finished product, of mold lines, mold surfaces and/or mold retention ribs creating localized areas of decreased impact resistance as well as the usual wall draft to permit removal of the product, or tub, from the forming mold. One primary disadvantage arising from this necessary wall draft concerns the mounting of the upper dishwasher rack supporting tracks in parallel.

In addition to the foregoing considerations which relate primarily to mounting and/or assembly procedures for large capacity front loading machines, the desirability of precluding tub discoloration and the desirability for firewall separation of the tub and those electrical components constituting a fire hazard are common to all automatic dishwashing machines. As regards plastic tub discoloration, it is preferred to avoid the use of certain conventional strand type reinforcing fillers, such as chopped strand fiber glass, for the reason that their intersection with the interior plastic surface may provide a "bleed path for food stains to permeate the tub structure.

The prior art suggestions relating to the use of small capacity plastic tubs of the top loading type, as in U. S. Pat. No. 2,691,986 and the use of separate plastic tub components, as in US. Pat. Nos. 2,877,778 and 3,385,306 offer little in the way of guidelines as regards a practical reduction to practice of a large capacity front loading tub for the reason that most of the problems requiring solution are not present in smaller and- /or top loading units and particularly those which employ an enveloping metal housing.

The present invention is concerned with the problem of impact resistance as deriving from the presence of mold retention ribs on the one piece molded tub walls while other of the aforementioned matters are more fully described and claimed in the following, commonly owned, copending applications:

Ser. No. 319,556, filed Dec. 29,, 1972, for Double,

Reversed Mounting Boss forPlastic Tubs;

Ser. No. 319,201, filed Dec. 29, 1972, for Fill Funnel Construction for Plastic Tubs;

Ser. No. 319,555, filed Dec. 29, 1972, for Thermal Growth Compensation and Mounting for Plastic Dishwasher Tubs;

Ser. No. 319,203, filed Dec. 29, 1972, for Front Loading Dishwasher Employing Plastic Tub Construction;

Ser. No. 319,202, filed Dec. 29, 1972, for Radiused Mold Walls for Plastic Tubs;

Ser. No. 317,368, filed Dec. 22, 1972, for Angled Levelling Foot for Domestic Appliance;

Ser. No. 319,200, filed Dec. 29, 1972, for Torsionally Reinforced, Skeletal Support Frame for Plastic Tubs;

Ser. No. 319,199, filed Dec. 29, 1972, for Plastic Tub Wall Alignment for Dishwasher Rack Mounting;

Ser. No. 319,348, filed Dec. 29, 1972 for Unit Handled Roller Assembly for Plastic Tub.

It is apparent that if the advantages in the elimination of the conventional front loading metal tub and the substitution therefor of a plastic tub, as regards decreased cost and weight, are to be retained the necessary mounting and support structure must not, in effect, comprise a confining tub support housing. Rather, a purpose of the invention is to provide an operative, front loading dishwasher unit having a plastic tub whose outer walls comprise the great majority of the outer dishwasher unit construction and would actually be exposed to view prior to installation of the dishwasher unit within an outer decorative housing, which decorative housing may comprise a free standing, portable unit or a permanent undercabinet installation. Accordingly, it is necessary that the tub, whose exemplary wall thickness is less than 0.2 inch, have a certain minimal impact resistance for factory handling and in use which latter resistance is prescribed by Underwriters Laboratory to be foot pounds.

SUMMARY OF THE INVENTION In the use of a multi-part contractible mold core for the formation of a unipartite plastic tub having a complexly configured bottom wall; the core must collapse away from the lower, complexly configured wall to permit withdrawal of the molded part from the core. Stated somewhat differently; the multipart contractible mold used to form the tub is contracted vertically to clear the lower mold part from the complex bottom configuration while the transverse dimensions of the tub forming mold core remain constant through the formation and removal of the tub. Thus the mold core sides used to form the tub sidewalls do not contract and product removal takes place through surface to surface sliding contact between the tub sidewalls and mold core.

In the particular three part mold core and tub design herein discussed the upper and lower mold core sections undergo vertical movement relative to each other and the stationary central core section during the collapsing movement. Accordingly, it is convenient to form the tub with internal mold retention ribs, concomitantly with the injection molding thereof, via slots formed on opposite sides of the central mold core section. The mold slots and internal tub ribs then cooperate to support the newly formed tub during the core contraction and tub removal process.

The presence, on the finished tub, of these internal mold retention ribs provide areas of decreased impact resistance which is compensated for by forming additional ribs, externally of the tub wall, whose width spans those of the inner ribs.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a broken front perspective of a front loading dishwasher employing a plastic tub;

FIG. 2 is a sectional view taken along line 2-2 of FIG. 1;

FIG. 3 is a schematic illustration of a plastic tub in the process of removal from a contractible mold core; and

FIGS. 4-6 are schematic illustrations of the manner in which impact stress concentrations are applied to variously configured shapes.

DESCRIPTION OF THE PREFERRED EMBODIMENT In FIG. 1 is illustrated a front loading dishwasher l0 employing a unipartite plastic tub 12 which includes a complexly configured lower wall 14 and opposed side-.

walls 16 having integral, inwardly extending, mold retention ribs 18 extending fore and aft of the tub along a major fraction of the tub depth. Impact protection ribs 20 are similarly integral with sidewalls 16 to extend outwardly thereof and whose length and sidewall location are coextensive with those of the inner mold retention ribs 18. The external ribs 20 have a greater width 22 than do the inner ribs so that with the longitudinal axes of the inner and outer ribs lying in a single plane, the width of the external ribs spans that of the internal ribs to compensate for the inherent decrease in impact resistance arising as a function of the necessary presence of the internal, mold retention ribs 18.

The reason for the required presence of internal ribs 18 will be readily understood from a brief description of the one piece molding procedures employed to form the tub. A three part contractible mold core 24 including a stationary central core section 26 flanked by relatively moveable upper and lower core sections 28, 30 in wedging engagement therewith cooperate with an appropriately configured female mold (not shown) to form the tub l2 a one piece injection molding. The schematic depiction of FIG. 3 illustrates the upper and lower core sections in their fully contracted positions, having undergone vertical movement relative to each other and the central core section from the phantom line position indicated by reference character 32, and the finished tub 12 in the process of being removed from the core. Central core section 26 is formed, on opposite sides thereof, with slots 34 whcih are in open communication with the injection molding chamber so that the internal tub retention ribs 18 are formed integral with the tub walls. The obvious purpose of these ribs is to maintain the tub in the position of FIG. 3 while core sections 28, 30 are being contracted prior to removal of the part from the core. The presence of such internal ribs, however, creates localized areas of decreased resistance to external impact for reasons that will become more apparent from the following description of FIGS. 4 and 5. In one sense and for explanatory purposes concerning the present invention; the conventional notch" effect may be thought of as a disruption in the otherwise unifonn flexibility of a test piece. Exemplary is a dual notch test as schematically depicted notches 36, which, each, represent a disruption of otherwise uniform test piece flexibility.

Similarly, with respect to the schematic depiction of FIG. 5 which involves the shape configuration with which the present invention is concerned, upstanding rib 38 introduces an area of greater rigidity into an otherwise uniformly flexible test piece 40 which, effectively, results in stress concentrations from an applied impact F being applied at the junctures 42, 44 of the rib sidewalls with the test piece.

In order to compensate for the decreased impact resistance resulting from such a ribbed constructionra rib 46 of greater width (FIG. 6) may be provided on the impact receiving side of the test piece. Thus impacts applied to the outer rib are primarily transmitted to the opposite test piece surface 48, outside, or laterally of, the juncture lines 42, 44' as indicated by the labeled lines of maximum stress in FIG. 6. The larger rib 46 acts, in effect, to spread the force application in such a manner as to span what would otherwise be the stress concentration lines (as along junctures 42, 44) absent the presence of external rib 46.

Accordingly, the female mold member, not shown, is appropriately configured to produce impact protection ribs whose longitudinal axes lie in the same plane with those of mold retention ribs 18 and whose width exceeds that of the mold retention ribs.

Although the present invention is concerned only with the tub wall rib configuration; per se, and it is thought that the foregoing brief discussion of the molding procedures employed provide adequate background information; reference may be had to the aforementioned copending application Ser. No. 319,202 for a more complete description of the molding procedures. I I

While the background discussion ofthis specification is related primarily to front-loading dishwashers of the type normally adapted for a permanent undercounter installation as opposed to the type of dishwasher having a hinged top closure for top-loading, it should be emphasized that many such machines are commercially produced and purchased for indefinite use as a freestanding model adaptable at a later time by the owner to a built-in unit beneath a kitchen countertop to blend with the matching cabinet structure of the kitchen. Generally, the difference between the unit designed as a *convertible" and the unit to be immediately permanently installed is the inclusion of side, rear and top cabinet appearance panels to the basic unit structure as designed for permanent installation. Therefore, it should be understood that the present invention is specifically directed to the front-loading type of dishwasher structure whether of the generally lower-cost unit having no appearance cabinet enclosure or the more elaborate, temporarily portable unit designed for later modification for use in a fixed location.

I claim:

1. In a unipartite, plastic tub fabricated in accordance with a molding process which results in said tub having an open front, a compound lower wall configuration, and opposed sidewalls including integral mold positioning ribs extending fore and aft of said tub internally thereof, the improvement comprising: said opposed sidewalls further including integral impact protection ribs extending outwardly of the external surfaces thereof and substantially coextensive in length and sidewall position with said positioning ribs; and said external ribs having a greater width than said internal ribs.

2. The plastic tub of claim 1 wherein the longitudinal axes of said internal and external ribs lie in a common plane.

3. A unipartite, plastic tub fabricated in accordance with a molding process which results in said tub having opposed sidewalls including integral mold positioning ribs extending inwardly thereof and lying between common parallel planes; impact protection ribs integral with and extending outwardly of said sidewalls; and said impact protection ribs being intersected by said parallel planes whereby impact stresses delivered to said impact protection ribs are transmitted tothe tub sidewalls outside the juncture of said sidewalls and mold positioning ribs.

4. The plastic tub of claim 3 wherein all of said ribs are substantially coextensive in length.

5. In a unipartite, plastic tub fabricated in accordance with a molding process which results in said tub having an open front end and top, bottom, side and rear walls, the improvement comprising: at least one rib integral with and extending away. from a surface of one of said walls; an impact protection rib integral with and extending away from the opposed surface of said one sidewall; and said impact protection rib having a greater width than said first named rib and substantially coextensive in length and wall position therewith. 

1. In a unipartite, plastic tub fabricated in accordance with a molding process which results in said tub having an open front, a compound lower wall configuration, and opposed sidewalls including integral mold positioning ribs extending fore and aft of said tub internally thereof, the improvement comprising: said opposed sidewalls further including integral impact protection ribs extending outwardly of the external surfaces thereof and substantially coextensive in length and sidewall position with said positioning ribs; and said external ribs having a greater width than said internal ribs.
 2. The plastic tub of claim 1 wherein the longitudinal axes of said internal and external ribs lie in a common plane.
 3. A unipartite, plastic tub fabricated in accordance with a molding process which results in said tub having opposed sidewalls including integral mold positioning ribs extending inwardly thereof and lying between common parallel planes; impact protection ribs integral with and extending outwardly of said sidewalls; and said impact protection ribs being intersected by said parallel planes whereby impact stresses delivered to said impact protection ribs are transmitted to the tub sidewalls outside the juncture of said sidewalls and mold positioning ribs.
 4. The plastic tub of claim 3 wherein all of said ribs are substantially coextensive in length.
 5. In a unipartite, plastic tub fabricated in accordance with a molding process which results in said tub having an open front end and top, bottom, side and rear walls, the improvement comprising: at least one rib integral with and extending away from a surface of one of said walls; an impact protection rib integral with and extending away from the opposed surface of said one sidewall; and said impact protection rib having a greater width than said first named rib and substantially coextensive in length and wall position therewith. 